Production of acetylene

ABSTRACT

PRODUCTION OF PURE ACETYLENE FROM CRACKED GAS, OBTAINED BY CRACKING LIQUID HYDROCARBONS BY, IN PARTICULAR, THE SUBMERGED-FLAME PROCESS, BY COMBINING A NUMBER OF PROCESS STAGES, IN WHICH THE C2 TO C5 HYDROCARBONS CONTAINED IN   THE CRUDE ACETYLENE ARE WASHED OUT WITH AN ORGANIC SOLVENT SUCH AS TOLUENE.

April 1, 1-972 BRUNNER EIAL 3,657,375

PRODUCTION OF ACETYLENE Filed Feb. 16, 1970 //V VE/V TORS ERWl/V BRU/V/VER POL F PL A 7'2 KURT TAG'L IEBER KURT WEI/VFURTER w' WM Q 1% 5 l i JE US. Cl. 260-679 A 2 Claims ABSTRACT OF THE DISCLOSURE Production of pure acetylene from cracked gas, obtained by cracking liquid hydrocarbons by, in particular, the submerged-flame process, by combining a number of process stages, in which the C to C hydrocarbonscontained in the crude acetylene are Washed out with anorganic solventsuch as toluene.

This invention relates to a process for the production of acetylene by thermally cracking liquid hydrocarbons and workingup the cracked gas by removing carbon dioxide therefrom and washing out the acetylene with a selecttive solvent, the resultant crude acetylene being purified in a combined condensing and washing stage.

Thermal cracking of hydrocarbons and immediate quenching of the cracked gas are known to provide a gas mixture comprising hydrogen, carbon monoxide, carbon dioxide, methane, ethylene, acetylene and higher hydrocarbons. Thermal cracking processes for the production of such acetylene-containing gas mixtures are described, for example, in Chem.-Ing.-Tech., 26, No. 5, 253 (1954), US. Pat. No. 3,047,371 and US. Pat. No. 2,984,695.

The most commonly used industrial method of thermally cracking methane provides a cracked gas containing relatively low proportions of higher hydrocarbons.

In a known process, the acetylene is removed from this acetylene-containing cracked gas, usually following the removal of carbon dioxide by a suitable washing operation, by washingthe gas with a solvent showing 's'electivity for acetylene. A particularly suitable solvent for this purpose is N-methylpyrrolidone. There is thus obnited States Patent 0 3,657,375 Patented Apr. 18, 1972 ice : and washing with a solvent showing selectivity for acetylcue, the crude acetylene thus obtained being purified in a further washing stage.

We have found that pure acetylene may be produced by combining the following process steps:

(a) Conventional thermal cracking of normally liquid hydrocarbons;

(b) Conventional washing to remove carbon dioxide and subsequent washing to remove acetylene by means of a solvent showing selectivity for acetylene and recovering a crude acetylene which contains hydrocarbons of more than 2 carbon atoms and may also contain water;

(c) Chilling the crude acetylene to temperatures of from approximately -10 to 30 C. to achieve substantial condensation of the higher hydrocarbons of 5 or more carbon atoms;

((1) Treating the remaining partly purified crude acetylene by countercurrent washing at temperatures ranging from 20 to 40 C. until virtually all of the hydrocarbons accompanying the acetylene have been washed out, using an organic washing liquid having a boiling range o from 50 to 150 C. and'a freezing point below 40 r a a (e) Heating the washing liquid laden with extracted hydrocarbons and acetylene to temperatures ranging from approximately 0 to 40 C. and recovering the gaseous acetylene in a degasifier and recycling it to the crude acetylene; and v (f) Separating the acetylene-free washing liquid in a fractionating column into pure washing liquid as bottoms and the extracted hydrocarbons as overhead.

Where a water-moist crude acetylene is to be purified in stage (c) care must be taken to ensure that no solid Water separates during chilling of the acetylene to temperatures ranging from 10 to 30 C., since such ice would lead to blockage of the condensation apparatus.

tained, particularly Where a cracked gas obtained by therrnally cracking methane is being worked up, an acetylene which contains only low proportions of impurities consisting of higher hydrocarbons, that is, hydrocarbons containing from 3 to 5 carbon atoms.

In the thermal cracking of liquid hydrocarbons, and more particularly in the cracking of crude oil or other oils containing high-boiling components, for example by the submerged-flame process, there are produced cracked gases which contain substantial amounts of components having similar solubility to acetylene in the selective solvent, with the result that a crude acetylene is produced which contains major amounts of higher hydrocarbons containing 3 to 5 carbon atoms as impurities, examples of such hydrocarbons being isobutene, isopentane, cisand trans-butene-2, propadiene, propyne and. butadiene-l,3. The total concentration of such impurities may be more than 20% molar. This crude acetylene, which also contains higher hydrocarbons, i.e. essentially mixtures of hydrocarbons containing more than 5 carbon atoms, is unsuitable for further processing in many cases, as unsaturated impurities may often cause undesirable side reactions.

It is thus an object of the invention to provide a process for the production of pure acetylene by the thermal crack- We have found that this may be realized in a simple manner by loading the moist crude acetylene with methanol vapor such that in the condensation stage higher hydrocarbons consistiug essentially of a mixture of hydrocarbons of more than 5 carbon atoms, methanol and water condense simultaneously in the liquid phase, separating the'condensate in a separating vessel into a hydrocarbon phase and a methanol/ water phase still containing small amounts of hydrocarbon and acetylene, distilling the latter phase into water as bottoms and methanol as overhead, and recirculating the desorbed acetylene together with the uncondensed methanol vapor to the con: densation stage. 1

More particularly, our new process for the production of pure acetylene is carried out by thermally cracking liquid hydrocarbons, particularly crude or heavy fuel oil, by the aforementioned submerged-flame process, that is, by effecting cracking with an oxygen-fed flame burning beneath the surface of said oil, to give a cracked gas containing the following components in the proportions stated below:

Percent molar remainder hydrocarbons of 3 or more carbon atoms.

This gas is first treated in known manner with an aqueous solution of the potassium salt of a-methylaminopropionic acid or, preferably, diethanolamine to wash out the carbon dioxide and hydrogen sulfide.

From the gas, now free of carbon dioxide and hydrogen sulfide, the acetylene is removed in known manner by washing the gas With a suitable solvent, such as dimethylformamide, butyrolactone, tetraethyl glycol dimethyl ether and preferably N-methylpyrrolidone. This washing process is described in detail in Chem.-Ing.-Tech., loc. cit. It is convenient to carry out this wash by first passing the gas through a pre-wash stage to remove the highly soluble diacetylene.

The acetylene is washed out in the main washing column. The pressures used are between 5 and 20 atmospheres and preferably between 10 and 15 atmospheres, while the temperatures range from 10 to 50 C. Less soluble components dissolved in the solvent are removed in a stripper by rising acetylene, preferably at a column pressure of from 1 to 2 atmospheres absolute,

Regeneration of the solvent is then carried out in a vacuum column under pressures ranging from 0.15 to 0.3 atmospheres, the temperature at the bottom of the column being from 100 to 140 C., preferably from 110 to 120 C.

After this washing stage the crude acetylene obtained has, for example, the following composition:

Percent molar C H 75.6 Propane 0.44 'Propadiene 4.18 Propyne 2.41 H O 6.5

remainder hydrocarbons of 4 or more carbon atoms.

This crude acetylene (see FIG. 1) is compressed in a blower 1 to a pressure of not more than 1.7 atmospheres absolute and chilled in heat exchangers, for example a combination of three different heat exchangers, to temperatures ranging from approximately 10 to 30 C., preferably to approximately -29 C. The first heat exchanger of the combination of coolers is advantageously a water-irrigated direct-cooling apparatus 2 filled with a packing and also serving as a dissociation barrier for the acetylene. In the second heat exchanger 3 of the combination the gas is countercurrent cooled by condensate and pure acetylene, while the remaining heat is extracted in the third heat exchanger 4 by refrigerating agents.

The crude acetylene usually contains water vapor acquired from a water wash following the wash with N- methylpyrrolidone. In order to prevent blockage of the coolers, freezing of the water vapor before the crude acetylene enters the last heat exchanger, in which it is cooled to temperatures below C., must be avoided. To this end the crude acetylene is loaded with methanol before it enters the cooler cooled by refrigerating agents. This may be effected, for example, by saturating the crude acetylene with methanol vapor in a saturator or, more simply, by adding at 5 a sufficient quantity of methanol vapor, usually from 1 to 4 and preferably from 2 to 3 molar proportions based on the amount of water vapor present.

The crude acetylene, now partly purified by condensation of impurities in the coolers, is then subjected to countercurrent washing 6 with an organic washing liquid to remove the hydrocarbons of 3 to 5 carbon atoms which contaminate the acetylene. Suitable Washing liquids are those which are inert under the reaction conditions, are preferably weakly polar, boil at temperatures above 80 C., freeze at temperatures below 40 C. and have low viscosity at temperatures ranging from 20 to 40 C. Examples of such organic liquids are aliphatic or aromatic hydrocarbons free from functional groups. Specific examples are n-heptane, n-octane, m-xylene, di-n-butyl ether and preferably toluene. Mixtures of such compounds are also very suitable.

The following description is taken in conjunction with the accompanying schematic drawing of the invention as disclosed.

The wash with the organic liquid boiling above C. is carried out in known types of apparatus, for example in a washing column, the crude acetylene being fed in at the bottom, whilst the organic liquid trickles down from the top. The dimensions of this washing column are selected in the usual manner so as to ensure that the hydrocarbons are washed out as completely as possible. The heat of solution liberated during the countercurrent wash is conveniently removed in coolers 7, 8. At the top of the washing column there is thus obtained a purified acetylene still containing the following impurities: about .10 p.p.m. of propylene, 5 p.p.m. of propadiene and propyne ano about 1,000 p.p.m. of the organic washing liquid.

The laden organic liquid discharged at the bottom of the washing column is heated to approximately 0 to 20 C. and degassed, preferably in a stripper 9, for the recovery of the dissolved acetylene. The gas thus obtained contains about 75% molar of acetylene and is recycled and mixed with the cracked gas after the latter has been freed of carbon dioxide and hydrogen sulfide but before it is subjected to selective washing. The washing liquid discharged from said stripper is heated to about C. in a heat exchanger 10 and then passed to a fractionating column 11. The resulting overhead product consists essentially of hydrocarbons of 3 to 5 carbon atoms and contains approximately 0.5% molar of acetylene.

The regenerated organic washing liquid is cooled in conventional heat exchangers 12 and recycled to the washing column.

The methanol added to ensure that the water is separated in the liquid state is conveniently also recovered. To this end the condensate obtained in the separator by cooling the crude acetylene is mixed with water and separated in a separating vessel 13 into a methanol/water phase and a hydrocarbon phase. Acetylene liberated during this stage is recycled through a degassing pipe to the second heat exchanger of the combination of coolers.

Methanol is isolated from the methanol/water phase, most conveniently by distillation in a column 14, and is then re-used in the condensation stage, Acetylene thus liberated is also recycled, together with a portion of unconcentrated methanol vapor, to the stream of crude acetylene.

We claim:

1. A process for the production of pure acetylene which comprises:

(a) thermally cracking normally liquid hydrocarbons in the presence of oxygen with subsequent quenching under conditions conducive to the formation of acetylene;

(b) washing the gaseous effluent from step (a) with an organic liquid showing selectivity for the absorption of acetylene and selected from the group consisting of dimethyl formamide, butyrolactone, tetraethyl dimethyl glycol ether and N-methylpyrrolidone, recovering from said organic liquid a crude acetylene containing hydrocarbons having 3 carbon atoms or more and traces of water;

(0) cooling said crude acetylene to temperatures ranging from 10" C. to 30 C. and thereby freezing out hydrocarbons of more than 5 carbon atoms from said crude acetylene;

'(d) removing from the gaseous efiluent obtained from step (c) substantially all of the hydrocarbons containing 3 to 5 carbon atoms accompanying the acetylene by washing said crude acetylene obtained from step (c) with toluene as the wash liquid at temperatures ranging from 20 C. to 40 C.;

(e) recovering dissolved acetylene from the toluene obtained from step (d) by heating said toluene to temperatures ranging from C. to 40 C. and recycling the recovered acetylene to the gaseous effluent from step (b); and

(f) distilling the toluene Wash liquid obtained from step (e), thereby producing an overhead product consisting of hydrocarbons and a bottoms product consisting of pure toluene which is recycled to step (d) as fresh wash liquid.

2. A process as claimed in claim 1 for the purification of a water-containing crude acetylene, which further comprises the steps of (ca) loading the gaseous eflluent from step (b) with methanol vapor and condensing out of said laden elfiuent a liquid mixture comprising methanol, water and a hydrocarbon mixture consisting essentially of hydrocarbons of more than 5 carbon atoms,

(0b) separating the liquid effluent from step (ca) in a separating vessel into a hydrocarbon phase and a methanol/water phase still containing traces of hydrocarbons and acetylene,

(cc) distilling said methanol/ water phase to give 'water as the bottoms product and methanol as the overhead product, and

(cd) recycling desor bed acetylene obtained from step (cc), said desorbed acetylene being laden with methanol vapor not condensed in said step (cc), to the condensation step (ca).

References Cited UNITED STATES PATENTS 2,891,633 6/1959 Morro et al. 260-679 3,034,272 5/ 1962 Griffin et a1. -65 2,183,148 12/1939 Murphree 260-605 3,405,192 1 0M968 Kl'uis et al. 260-679 3,330,124 7/ 1967 Marshall 208-187 3,174,292 3/1969 Kasbobm et a1. 62-17 3,531,915 610/1970 Nagel et al. 55-32 2,894,602 7/1959 \Fauser 62-17 FOREIGN PATENTS 573,349 3/ 1959 Canada 55-64 834,419 5/1960 Great Britain 260-679 A =DELBEERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner 11.8. C1. X.R. 62-17, 20

mg UNITED STATES PATENT oFTTcE CERTIFICATE OF- CORRECNCN Patent No. 5, 57,575 Dated Ap i 97 Inventor(g) Erwin Brunner et a1 It is certified that error appears in the above-identified patent and that saidLettera Patent are hereby corrected as shown below:

Column 1, line 56, "2,984,695" should read 2,985,695

Column 5, line 41, -29 C" should read -28 C Column 4, line 18, "ano" should read and line 19, insert The following description is taken in conjunction with the accompanying schematic drawing of the invention as disclosed.

Signed and sealed this 5th day of September 1972.

(SEAL) Attest:

EDWARD M.FLETCHER JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

